Re-evaluating photoluminescent defects in Cu$_2$O
Defects in cuprous oxide (Cu$_2$O) strongly influence its performance in applications ranging from photovoltaics to emerging quantum technologies based on Rydberg excitons where microscopic crystal purity is essential. Photoluminescence (PL) spectroscopy is widely used as a diagnostic of material quality yet the origins of the sub-band-gap PL lines remain controversial despite decades of study. Using density functional theory we systematically evaluate native point defects in Cu$_2$O and identify which produce robust electronic states within the band gap. By combining supercell-size convergence criteria and cross-functional consistency checks we show that the widely accepted assignments of the 1.35\,eV, 1.5\,eV, and 1.7\,eV PL lines to copper and oxygen vacancies are unsupported. Instead we find that oxygen interstitials, copper interstitials and one form of split copper vacancy are the only native defects that consistently introduce true in-gap states. These results provide a revised framework for interpreting PL spectra and offer a more reliable basis for diagnosing crystal quality in Cu$_2$O-based quantum-device platforms.